The objective of this study was to compare the effects of ammonium and nitrate sources of nitrogen on the growth and chemical composition of rye (Scale cereale) in the presence and absence of potassium ethylxanthate (KEtX) as a nitrification inhibitor, and the effects of acidity and metal ions on nitrification in soils. Results showed that application of nitrogen caused a significant increase in dry matter yield and offtakes of N and Mn but reduced P and Fe contents at various levels. Addition of the inhibitor increased N and Mn content and offtake, and decreased P level and offtake significantly. Different forms of nitrogen showed no significant differences either in foliage yield or nutrient content except stimulation of Fe uptake by nitrate-N. Ca, Mg, K and nitrate-N in tissues remained practically unaffected. Nitrification in acid peat was stimulated by adding lime but suppressed by Cd as sulphate and Al, Fe, Mn, Co, Ni, Cu and Zn when added as chlorides. Generally, inhibition increased with metal addition. Al (200 μg g⁻¹), Fe and Mn (100 μg g⁻¹) promoted nitrification. Addition of all the metals up to 3000 μg g⁻¹ peat depressed the process and the extent of suppression depends on the type and concentration of metallic ions. Among the metallic ions studied, Cd was the most toxic. The critical concentrations of Cd; Al; and Fe, Mn, Co, Ni, Cu and Zn were 500, 1000 and 2000 μg g⁻¹ peat respectively. However, results provide evidence that nitrification can proceed slowly at pH 3.4 to 4.0 in the absence of metals. Although nitrification was suppressed, general microbial activity was not repressed even in the presence of 3000 pg Al g⁻¹ peat.